The disclosure relates generally to voltage boost systems.
A voltage boost circuit or charge pump is an electronic circuit that uses capacitors for energy storage to create a higher voltage power source. One challenge with charge pumps is that when creating a higher voltage power source, such as in a three times a supply voltage (3×Vdd) charge pump, voltages may be generated in excess of the oxide-stress limit of a field effect transistor (FET), i.e., a stress limit of the gate oxide thickness. Previous approaches have used FET devices with an oxide stress limit greater than the output voltage of the pumping system. This situation forces inclusion of a thicker, and typically lower performance FET in a technology menu which adds cost and complexity such as additional mask steps and extra characterization requirements. For example, a “medium” gate oxide thickness FET may have a gate oxide of approximately 22 Angstroms (Å), while a thicker gate oxide, lower performance FET may require a gate oxide of approximately 52 Å. Gate voltage controllers for generating a safe gate drive level below an excessive stress level of the oxide have been implemented, but they are limited in terms of the amount of boost permissible and require a precision current source for calibration.
Another issue with charge pumps is that an output voltage will rise to levels in excess of the regulated drain (output) voltage of an output passgate. The typical approach to address this problem is tying the n-well of the passgate to the source or drain thereof. However, this creates a forward-biased junction and turns on a parasitic PNP device, rendering the charge pump inoperable.